Over the last couple of decades broad acre spraying has become an essential component of crop management on most farms in Australia. Chemical companies have developed new chemical pesticides for killing weeds, insect pests and diseases which attack cash crops. The most commonly used technique for broad acre spraying of pesticides is the use of boom sprays, which may be self-propelled or towed behind another vehicle. A typical boom spray has a plurality of spray nozzles mounted at spaced locations along a boom, a large tank for containing the spray liquid and a pump system for pumping the liquid to the nozzles. A control system is usually provided for controlling the rate at which the pesticide is sprayed relative to ground speed, and a marker system may be used to aid driving and avoid overlapping.
One of the disadvantages of conventional boom sprays is that herbicides are sprayed indiscriminately on the crop, bare ground and weeds. This is of concern in the case of food crops, with consumer groups becoming increasingly vocal about chemical residue in crops and livestock. There is also an economic disincentive since a much greater volume of chemical spray must be applied per hectare than is actually required to effectively control the weeds.
Co-pending Australian patent application No. 16482/99 describes a method and device for discriminating different types of ground vegetation in agriculture in which an artificial light source is employed for directing a beam of light onto the vegetation. A sensor assembly is provided for detecting reflected radiation from the vegetation in a selected wavelength band and generating a sensing signal in response to the detection. A logic controller determines whether a magnitude of the sensing signal falls within a predetermined range of values in order to distinguish one type of vegetation from another type. In the red and near infrared wavelengths there is some variation in reflectance from one plant to another, depending on a number of factors. The logic controller compares the amplitude of the sensing signal with a decision window. If the amplitude of the incoming sensing signal falls within the decision window then the logic controller determines that a certain type of plant has been detected and activates the corresponding solenoid valve to deliver a dose of spray liquid from a spray nozzle to the targeted weed.
With certain types of vegetation, it is not possible to distinguish weed from crop plants based on the sensing of the reflected radiation alone. Therefore, AU 16482/99 also describes the use of a plurality of sensors arranged in an array with a geometric configuration adapted to aid in discriminating different types of vegetation. For example, a linear array of three sensors can be adapted to distinguish a stem type target vegetation such as, for example, skeleton weed (Chondrilla juncea L.). A unique decision window for each sensor in the array is programmed into the logic controller in order to detect sensing signals which fall within respective decision windows. By a judicious choice of geometric array of sensors and providing a unique decision window for each sensor, the logic controller can accurately distinguish between different shaped plants.
Whilst the above arrangement of sensors in a geometric array works very well, in practice the need to change the geometric array for each type of weed desired to be distinguished is rather inconvenient.